Active dried yeast is prepared in a dry, granular form by drying extruded, compressed yeast under carefully controlled conditions to a moisture content of about 7.5 to 8.5%. An advantage with active, dry yeast is that it does not require refrigeration to maintain its activity, and the shelf life can vary from one to twelve months, depending upon ambient storage conditions, the stability being inversely related to storage temperature and moisture content.
There has recently been developed an active dried bakers' yeast which is said to have considerably higher fermentative activity than traditional active dry yeasts. It is also said to have an oblong porous structure which permits its use in doughs without prior rehydration. The yeast is marketed by Gist-Brocades U.S.A., Inc. under the trademark Fermipan, as an instant active dried yeast and is described in U.S. Pat. Nos. 4,217,420; 3,843,800; and 4,341,871. The yeast is normally packaged under vacuum, and once the package is opened, the yeast has to be used within three days. When premixed with flour, special precautions have to be taken to insure minimum contact of the yeast with moisture. Vacuum packing or packing in a nitrogen atmosphere is also recommended for the dry mixes containing the yeast.
Prior U.S. Pat. No. 2,894,842 to Mitchell et al, refers to the improved thermostability and storage stability achieved by drying to lower than 7.5 to 8.5% moisture content, for instance about 3-4%. U.S. Pat. No. 2,894,842 is directed to the problem of rehydrating yeast having such low moisture content, and teaches adding a lipophilic substance such as an aliphatic partial ester of sorbitan to the yeast prior to dehydration.
In a paper by Chen et al, entitled "Active Dry Yeast: Protection Against Oxidative Deterioration During Storage", Food Technology, December, 1966, Volumn 20, No. 12, pages 79-83, it is pointed out that even at the lower levels of 3-4% moisture content, the deteriorative effect of oxygen is not eliminated.
Prior U.S. Pat. No. 2,523,483 describes coating yeast with shortening to preserve the yeast. Examples of shortening described in the patent are lard and animal fat, preferably hydrogenated fat. The dried yeast is granulated to a size about the same as sugar, and to a moisture content such that it feels dry. The dried yeast is then mixed with the fat, either by hand or in a cake mixer, or the equivalent. The shortening can be in a melted or solid condition, and mixing is carried out to ensure that each yeast granual is completely covered with fat.
The problem with the use of a solid fat or shortening is that the same is not cold-water soluble, and, as stated in U.S. Pat. No. 2,523,483, warm water should be added to the coated yeast at a sufficiently high temperature to quickly melt the fat from the yeast and free the yeast particles to permit them to hydrate and commence fermentation.
In this regard, it is critical to avoid exposing the yeast, either at the time of application of a coating or at the time of removal of the coating, to temperatures so high so as to kill at least a part of the yeast. In U.S. Pat. No. 2,523,483, the yeast was not exposed to the high temperatures of molten lard or animal fat during coating, but rather coating was carried out by the cruder or less sophisticated method of simple mixing with the plastic material.
In U.S. Pat. No. 2,523,483, it is also pointed out that the yeast, during removal of the coating, is protected from the hot water by the coating, the implication being that some yeast could be killed even from the temperature of water which is too hot.
It has also been proposed to coat water-sensitive materials with aqueous-based coatings, under carefully controlled conditions, but the problem here is that even with the carefully controlled conditions, the yeast in the coating process would pick up too much moisture, adversely affecting its shelf life or stability.